Link Layer

Link Layer

Where we are in the Course

• Moving on up to the Link Layer!

CSE 461 University of Washington 2

Physical

Network

Transport

Application

Scope of the Link Layer

• Concerns how to transfer messages over one or more connected links

• Messages are frames, of limited size• Builds on the physical layer

In terms of layers …

Actual data path

Virtual data path

Network

Physical

In terms of layers (2)

Actual data path

Virtual data path

Network

Physical

Typical Implementation of Layers (2)

Topics

1. Framing• Delimiting start/end of frames

2. Error detection and correction• Handling errors

3. Retransmissions• Handling loss

4. Multiple Access• 802.11, classic Ethernet

5. Switching• Modern Ethernet

FramingDelimiting start/end of frames

• The Physical layer gives us a stream of bits. How do we interpret it as a sequence of frames?

…10110 …

Framing Methods

• We’ll look at:• Byte count (motivation)

• Byte stuffing

• Bit stuffing

• In practice, the physical layer often helps to identify frame boundaries• E.g., Ethernet, 802.11

Simple ideas?

Byte Count

• First try:• Let’s start each frame with a length field!

• It’s simple, and hopefully good enough …

Byte Count (2)

• How well do you think it works?

Byte Count (3)

• Difficult to re-synchronize after framing error• Want a way to scan for a start of frame

Byte Stuffing

• Better idea:• Have a special flag byte value for start/end of frame• Replace (“stuff”) the flag with an escape code• Problem?

Byte Stuffing

• Better idea:• Have a special flag byte value for start/end of frame• Replace (“stuff”) the flag with an escape code• Complication: have to escape the escape code too!

Byte Stuffing (2)

• Rules:• Replace each FLAG in data with ESC FLAG

• Replace each ESC in data with ESC ESC

Byte Stuffing (3)

• Now any unescaped FLAG is the start/end of a frame

Unstuffing

You see:

1. Solitary FLAG?

2. Solitary ESC?

3. ESC FLAG?

4. ESC ESC FLAG?

5. ESC ESC ESC FLAG?

6. ESC FLAG FLAG?

Unstuffing

You see:

1. Solitary FLAG? -> Start or end of packet

2. Solitary ESC? -> Bad packet!

3. ESC FLAG? -> remove ESC and pass FLAG through

4. ESC ESC FLAG? -> removed ESC and then start or end of packet

5. ESC ESC ESC FLAG? -> pass ESC FLAG through

6. ESC FLAG FLAG? -> pass FLAG through then start or end of packet

Bit Stuffing

• Can stuff at the bit level too• Call a flag six consecutive 1s

• On transmit, after five 1s in the data, insert a 0

• On receive, a 0 after five 1s is deleted

Bit Stuffing (2)

• Example:

Transmitted bitswith stuffing

Data bits

Bit Stuffing (3)

• So how does it compare with byte stuffing?

Transmitted bitswith stuffing

Data bits

Link Example: PPP over SONET

• PPP is Point-to-Point Protocol

• Widely used for link framing• E.g., it is used to frame IP packets that are sent over SONET optical links

Link Example: PPP over SONET (2)

• Think of SONET as a bit stream, and PPP as the framing that carries an IP packet over the link

Protocol stacksPPP frames may be split over

SONET payloads

Link Example: PPP over SONET (3)

• Framing uses byte stuffing

• FLAG is 0x7E and ESC is 0x7D

Link Layer: Error detection and correction

• Some bits will be received in error due to noise. What can we do?

• Detect errors with codes• Retransmit lost frames• Correct errors with codes

•Reliability is a concern that cuts across the layers

Problem – Noise may flip received bits

Signal0 0 0 0

0 0 0 0

SlightlyNoisy

Verynoisy

Approach – Add Redundancy

• Error detection codes• Add check bits to the message bits to let some errors be

detected

• Error correction codes• Add more check bits to let some errors be corrected

• Key issue is now to structure the code to detect many errors with few check bits and modest computation

• Ideas?

Motivating Example

• A simple code to handle errors:• Send two copies! Error if different.

• How good is this code?• How many errors can it detect/correct?• How many errors will make it fail?

Motivating Example (2)

• We want to handle more errors with less overhead• Will look at better codes; they are applied mathematics• But, they can’t handle all errors• And they focus on accidental errors (will look at secure

hashes later)

Using Error Codes

• Codeword consists of D data plus R check bits (=systematic block code)

• Sender: • Compute R check bits based on the D data bits; send the

codeword of D+R bits

D R=fn(D)

Data bits Check bits

Using Error Codes (2)

• Receiver: • Receive D+R bits with unknown errors• Recompute R check bits based on the D data bits; error if

R doesn’t match R’

D R’

Data bits Check bits

R=fn(D)=?

Intuition for Error Codes

• For D data bits, R check bits:

• Randomly chosen codeword is unlikely to be correct; overhead is low

Allcodewords

Correctcodewords

R.W. Hamming (1915-1998)

• Much early work on codes:• “Error Detecting and Error Correcting

Codes”, BSTJ, 1950

• “If the computer can tell when an error has occurred, surely there is a way of telling where the error is so the computer can correct the error itself” - Hamming

Hamming Distance

• Hamming distance between two codes (D1 D2) is the number of bit flips needed to change D1 to D2

• Hamming distance of a coding is the minimum error distance between any pair of codewords (bit-strings) that cannot be detected

Hamming Distance (2)

• Error detection:• For a coding of distance d+1, up to d errors will always be

detected

• Error correction:• For a coding of distance 2d+1, up to d errors can always be

corrected by mapping to the closest valid codeword

Simple Error Detection – Parity Bit

• Take D data bits, add 1 check bit that is the sum of the D bits

• Sum is modulo 2 or XOR

Parity Bit (2)

• How well does parity work?• What is the distance of the code?• How many errors will it detect/correct?

• What about larger errors?

Checksums

• Idea: sum up data in N-bit words• Widely used in, e.g., TCP/IP/UDP

• Stronger protection than parity

1500 bytes 16 bits

Internet Checksum

• Sum is defined in 1s complement arithmetic (must add back carries)

• And it’s the negative sum

• “The checksum field is the 16 bit one's complement of the one's complement sum of all 16 bit words …” – RFC 791

Internet Checksum (2)Sending:

1.Arrange data in 16-bit words

2.Put zero in checksum position, add

3.Add any carryover back to get 16 bits

4.Negate (complement) to get sum

0001 f204 f4f5 f6f7

+(0000)------2ddf0

ddf0 + 2 ------

Internet Checksum (3)0001 f204 f4f5 f6f7

+(0000)------2ddf1

ddf1 + 2 ------

Sending:

1.Arrange data in 16-bit words

2.Put zero in checksum position, add

3.Add any carryover back to get 16 bits

4.Negate (complement) to get sum

Internet Checksum (4)Receiving:

1. Arrange data in 16-bit words

2. Checksum will be non-zero, add

3. Add any carryover back to get 16 bits

4. Negate the result and check it is 0

0001 f204 f4f5 f6f7

+ 220c ------2fffd

fffd + 2 ------

Internet Checksum (5)Receiving:

1. Arrange data in 16-bit words

2. Checksum will be non-zero, add

3. Add any carryover back to get 16 bits

4. Negate the result and check it is 0

0001 f204 f4f5 f6f7

+ 220c ------2fffd

fffd + 2 ------

Internet Checksum (6)

• How well does the checksum work?• What is the distance of the code?• How many errors will it detect/correct?

• What about larger errors?

Cyclic Redundancy Check (CRC)

• Even stronger protection• Given n data bits, generate k check bits such that the n+k

bits are evenly divisible by a generator C

• Example with numbers:• n = 302, k = one digit, C = 3

CRCs (2)

• The catch:• It’s based on mathematics of finite fields, in which “numbers” represent

polynomials

• e.g, 10011010 is x7

• What this means:• We work with binary values and operate using modulo 2 arithmetic

CRCs (3)

• Send Procedure:

1. Extend the n data bits with k zeros

2. Divide by the generator value C

3. Keep remainder, ignore quotient

4. Adjust k check bits by remainder

• Receive Procedure:

1. Divide and check for zero remainder

CRCs (4)

Data bits:1101011111

Check bits:C(x)=x4+x1+1

C = 10011k = 4

CRCs (5)

CSE 461 University of Washington 53Plus

CRCs (6)

• Protection depend on generator• Standard CRC-32 is 10000010 01100000 10001110 110110111

• Properties:• HD=4, detects up to triple bit errors

• Also odd number of errors

• And bursts of up to k bits in error

• Not vulnerable to systematic errors like checksums

Why Error Correction is Hard

• If we had reliable check bits we could use them to narrow down the position of the error

• Then correction would be easy

• But error could be in the check bits as well as the data bits!

• Data might even be correct

Intuition for Error Correcting Code

• Suppose we construct a code with a Hamming distance of at least 3

• Need ≥3 bit errors to change one valid codeword into another

• Single bit errors will be closest to a unique valid codeword

• If we assume errors are only 1 bit, we can correct them by mapping an error to the closest valid codeword

• Works for d errors if HD ≥ 2d + 1

Intuition (2)

• Visualization of code:

Validcodeword

Errorcodeword

Intuition (3)

• Visualization of code:

Validcodeword

Errorcodeword

Single bit errorfrom A

Three bit errors to get to B

Hamming Code

• Gives a method for constructing a code with a distance of 3

• Uses n = 2k – k – 1, e.g., n=4, k=3• Put check bits in positions p that are powers of 2, starting

with position 1• Check bit in position p is parity of positions with a p term

in their values

• Plus an easy way to correct [soon]

Hamming Code (2)

• Example: data=0101, 3 check bits• 7 bit code, check bit positions 1, 2, 4• Check 1 covers positions 1, 3, 5, 7• Check 2 covers positions 2, 3, 6, 7• Check 4 covers positions 4, 5, 6, 7

_ _ _ _ _ _ _1 2 3 4 5 6 7

Hamming Code (3)

• Example: data=0101, 3 check bits• 7 bit code, check bit positions 1, 2, 4• Check 1 covers positions 1, 3, 5, 7• Check 2 covers positions 2, 3, 6, 7• Check 4 covers positions 4, 5, 6, 7

0 1 0 0 1 0 1

p1= 0+1+1 = 0, p2= 0+0+1 = 1, p4= 1+0+1 = 0

1 2 3 4 5 6 7

Hamming Code (4)

• To decode:• Recompute check bits (with parity sum including the

check bit)• Arrange as a binary number• Value (syndrome) tells error position• Value of zero means no error• Otherwise, flip bit to correct

Hamming Code (5)

• Example, continued

0 1 0 0 1 0 1

p1= p2= p4=

Syndrome = Data =

1 2 3 4 5 6 7

Hamming Code (6)

0 1 0 0 1 0 1

p1= 0+0+1+1 = 0, p2= 1+0+0+1 = 0,p4= 0+1+0+1 = 0

Syndrome = 000, no errorData = 0 1 0 1

1 2 3 4 5 6 7

Hamming Code (7)

0 1 0 0 1 1 1

p1= p2= p4=

Syndrome = Data =

1 2 3 4 5 6 7

Hamming Code (8)

0 1 0 0 1 1 1

p1= 0+0+1+1 = 0, p2= 1+0+1+1 = 1,p4= 0+1+1+1 = 1

Syndrome = 1 1 0, flip position 6Data = 0 1 0 1 (correct after flip!)

1 2 3 4 5 6 7

Hamming Code (3)

• Example: bad message 0100111• 7 bit code, check bit positions 1, 2, 4• Check 1 covers positions 1, 3, 5, 7• Check 2 covers positions 2, 3, 6, 7• Check 4 covers positions 4, 5, 6, 7

0 1 0 0 1 1 1

p1= 0+0+1+1 = 0, p2= 1+0+1+1 = 1, p4= 0+1+1+1 = 1

1 2 3 4 5 6 7

Hamming Code (3)

• Example: bad message 0100111• 7 bit code, check bit positions 1, 2, 4• Check 1 covers positions 1, 3, 5, 7• Check 2 covers positions 2, 3, 6, 7• Check 4 covers positions 4, 5, 6, 7

0 1 0 0 1 1 1

p1= 0+0+1+1 = 0, p2= 1+0+1+1 = 1, p4= 0+1+1+1 = 1

1 2 3 4 5 6 7

Other Error Correction Codes

• Real codes are more involved than Hamming

• E.g., Convolutional codes (§3.2.3)• Take a stream of data and output a mix of the input bits• Makes each output bit less fragile• Decode using Viterbi algorithm (which can use bit confidence

values)

Other Codes (2) – Turbo Codes • Turbo Codes

• Evolution of convolutional codes• Sends multiple sets of parity bits with payload• Decodes sets together (e.g. Sudoku)• Used in 3G and 4G cellular technologies

• Invented and patented by Claude Berrou• Professor at École Nationale Supérieure des

Télécommunications de Bretagne

Other Codes (3) – LDPC • Low Density Parity Check (§3.2.3)

• LDPC based on sparse matrices• Decoded iteratively using a belief

propagation algorithm

• Invented by Robert Gallager in 1963 as part of his PhD thesis

• Promptly forgotten until 1996 …

More coding theory • This is a huge field. • See EE 505, 514, 515 for more info

− These are graduate classes• Key points:

− Coding allows us to detect and correct bit errors received from the PHY

− It is very complicated. Abstract away with Hamming Distance

Detection vs. Correction

• Which is better will depend on the pattern of errors. For example:

• 1000 bit messages with a bit error rate (BER) of 1 in 10000

• Which has less overhead?

Detection vs. Correction

• Which is better will depend on the pattern of errors. For example:

• 1000 bit messages with a bit error rate (BER) of 1 in 10000

• Which has less overhead?• It still depends! We need to know more about the errors

Detection vs. Correction (2)

Assume bit errors are random• Messages have 0 or maybe 1 error (1/10 of the time)

Error correction: • Need ~10 check bits per message

• Overhead:

Error detection: • Need ~1 check bits per message plus 1000 bit retransmission

• Overhead:

Assume errors come in bursts of 100• Only 1 or 2 messages in 1000 have significant (multi-bit) errors

Error correction: • Need >>100 check bits per message

• Overhead:

Error detection: • Need 32 check bits per message plus 1000 bit resend 2/1000 of the time

• Overhead:

• Error correction: • Needed when errors are expected

• Or when no time for retransmission

• Error detection: • More efficient when errors are not expected

• And when errors are large when they do occur

Error Correction in Practice

• Heavily used in physical layer• LDPC is the future, used for demanding links like 802.11, DVB, WiMAX, power-line, …

• Convolutional codes widely used in practice

• Error detection (w/ retransmission) is used in the link layer and above for residual errors

• Correction also used in the application layer• Called Forward Error Correction (FEC)

• Normally with an erasure error model

• E.g., Reed-Solomon (CDs, DVDs, etc.)

Link Layer: Retransmissions

Context on Reliability

• Where in the stack should we place reliability functions?

Physical

Network

Transport

Application

Context on Reliability (2)

• Everywhere! It is a key issue• Different layers contribute differently

Recover actions(correctness)

Mask errors(performance optimization)

Physical

Network

Transport

Application

So what do we do if a frame is corrupted?

• From sender?

• From receiver?

ARQ (Automatic Repeat reQuest)

• ARQ often used when errors are common or must be corrected

• E.g., WiFi, and TCP (later)

• Rules at sender and receiver:• Receiver automatically acknowledges correct frames with

an ACK• Sender automatically resends after a timeout, until an ACK

is received

ARQ (2)

• Normal operation (no loss)

ACKTimeout Time

Sender Receiver

ARQ (3)

• Loss and retransmission

Timeout Time

Sender Receiver

So What’s Tricky About ARQ?

Duplicates

• What happens if an ACK is lost?

ACKTimeout

Sender Receiver

Duplicates (2)

• What happens if an ACK is lost?

ACKTimeout

Sender Receiver

New Frame??

Duplicates (3)

• Or the timeout is early?

Timeout

Sender Receiver

Duplicates (4)

• Or the timeout is early?

Timeout

Sender Receiver

New Frame??

So What’s Tricky About ARQ?

• Two non-trivial issues:• How long to set the timeout? • How to avoid accepting duplicate frames as new frames

• Want performance in the common case and correctness always

• Ideas?

Timeouts

• Timeout should be:• Not too big (link goes idle)

• Not too small (spurious resend)

• Fairly easy on a LAN• Clear worst case, little variation

• Fairly difficult over the Internet• Much variation, no obvious bound

• We’ll revisit this with TCP (later)

Sequence Numbers

• Frames and ACKs must both carry sequence numbers for correctness

• To distinguish the current frame from the next one, a single bit (two numbers) is sufficient

• Called Stop-and-Wait

Stop-and-Wait

• In the normal case:

Sender Receiver

Stop-and-Wait (2)

• In the normal case:

Frame 0

ACK 0Timeout Time

Sender Receiver

Frame 1

Stop-and-Wait (3)

• With ACK loss:

Frame 0

ACK 0Timeout

Sender Receiver

Stop-and-Wait (4)

• With ACK loss:

Frame 0

ACK 0Timeout

Sender Receiver

It’s a Resend!

Stop-and-Wait (5)

• With early timeout:

Frame 0

Timeout

Sender Receiver

Stop-and-Wait (6)

• With early timeout:

Frame 0

Timeout

Sender Receiver

It’s aResend

OK …

Limitation of Stop-and-Wait

• It allows only a single frame to be outstanding from the sender:

• Good for LAN, not efficient for high BD

• Ex: R=1 Mbps, D = 50 ms• How many frames/sec? If R=10 Mbps?

Sliding Window

• Generalization of stop-and-wait• Allows W frames to be outstanding• Can send W frames per RTT (=2D)

• Various options for numbering frames/ACKs and handling loss• Will look at along with TCP (later)

Multiple Access

• Multiplexing is the network word for the sharing of a resource

• What are some obvious ways to multiple a resource?

• Multiplexing is the network word for the sharing of a resource

• Classic scenario is sharing a link among different users• Time Division Multiplexing (TDM)

• Frequency Division Multiplexing (FDM)

Time Division Multiplexing (TDM)

•Users take turns on a fixed schedule

2 2 2 2

Frequency Division Multiplexing (FDM)

• Put different users on different frequency bands

Overall FDM channel

TDM versus FDM

• Tradeoffs?

TDM versus FDM (2)

• In TDM a user sends at a high rate a fraction of the time; in FDM, a user sends at a low rate all the time

TimeFDM

TDM/FDM Usage

• Statically divide a resource• Suited for continuous traffic, fixed number of users

• Widely used in telecommunications• TV and radio stations (FDM)• GSM (2G cellular) allocates calls using TDM within FDM

Multiplexing Network Traffic

• Network traffic is bursty• ON/OFF sources • Load varies greatly over time

TimeRate

Multiplexing Network Traffic (2)

• Network traffic is bursty• Inefficient to always allocate user their ON needs with

TDM/FDM

TimeRate

Multiplexing Network Traffic (3)

• Multiple access schemes multiplex users according to demands – for gains of statistical multiplexing

TimeRate

R’<2R

Two users, each need RTogether they need R’ < 2R

How to control?

Two classes of multiple access algorithms: Centralized and distributed

• Centralized: Use a privileged “Scheduler” to pick who gets to transmit and when.

• Positives: Scales well, usually efficient.• Negatives: Requirements management, fairness• Examples: Cellular networks (tower coordinates)

• Distributed: Have all participants “figure it out” through some mechanism.

• Positives: Operates well under low load, easy to set up, equality• Negatives: Scaling is really hard, • Examples: Wifi networks

Distributed (random) Access

• How do nodes share a single link? Who sends when, e.g., in WiFI?

• Explore with a simple model

• Assume no-one is in charge• Distributed system

Distributed (random) Access (2)

• We will explore random multiple access control(MAC) protocols

• This is the basis for classic Ethernet• Remember: data traffic is bursty

Zzzz..Busy! Ho hum

ALOHA Network

• Seminal computer network connecting the Hawaiian islands in the late 1960s

• When should nodes send?• A new protocol was devised by

Norm Abramson …Hawaii

ALOHA Protocol

• Simple idea:• Node just sends when it has traffic. • If there was a collision (no ACK received) then wait a

random time and resend

• That’s it!

ALOHA Protocol (2)

• Some frames will be lost, but many may get through…

• Limitations?

ALOHA Protocol (3)

• Simple, decentralized protocol that works well under low load!

• Not efficient under high load• Analysis shows at most 18% efficiency

• Improvement: divide time into slots and efficiency goes up to 36%

• We’ll look at other improvements

Classic Ethernet • ALOHA inspired Bob Metcalfe to

invent Ethernet for LANs in 1973• Nodes share 10 Mbps coaxial cable• Hugely popular in 1980s, 1990s

CSMA (Carrier Sense Multiple Access)

• Improve ALOHA by listening for activity before we send (Doh!)

• Can do easily with wires, not wireless

• So does this eliminate collisions?• Why or why not?

CSMA (2)

• Still possible to listen and hear nothing when another node is sending because of delay

CSMA (3)

• CSMA is a good defense against collisions only when BD is small

CSMA/CD (with Collision Detection)

• Can reduce the cost of collisions by detecting them and aborting (Jam) the rest of the frame time

• Again, we can do this with wires

X X X X X X X XJam! Jam!

CSMA/CD Complications

• Everyone who collides needs to know it happened• How long do we need to wait to know there wasn’t a JAM?

CSMA/CD Complications

• Everyone who collides needs to know it happened• How long do we need to wait to know there wasn’t a JAM?• Time window in which a node may hear of a collision

(transmission + jam) is 2D seconds

CSMA/CD Complications (2)

• Impose a minimum frame length of 2D seconds• So node can’t finish before collision• Ethernet minimum frame is 64 bytes – Also sets maximum

network length (500m w/ coax, 100m w/ Twisted Pair)

CSMA “Persistence”

• What should a node do if another node is sending?

• Idea: Wait until it is done, and send

What now?

CSMA “Persistence” (2)

• Problem is that multiple waiting nodes will queue up then collide

• More load, more of a problem

Now! Now!Uh oh

• Problem is that multiple waiting nodes will queue up then collide

• Ideas?

Now! Now!Uh oh

• Intuition for a better solution• If there are N queued senders, we want each to send next

with probability 1/N

Send p=½WhewSend p=½

Binary Exponential Backoff (BEB)

• Cleverly estimates the probability• 1st collision, wait 0 or 1 frame times• 2nd collision, wait from 0 to 3 times• 3rd collision, wait from 0 to 7 times …

• BEB doubles interval for each successive collision• Quickly gets large enough to work• Very efficient in practice

Classic Ethernet, or IEEE 802.3

• Most popular LAN of the 1980s, 1990s• 10 Mbps over shared coaxial cable, with baseband signals• Multiple access with “1-persistent CSMA/CD with BEB”

Ethernet Frame Format

• Has addresses to identify the sender and receiver

• CRC-32 for error detection; no ACKs or retransmission

• Start of frame identified with physical layer preamble Packet from Network layer (IP)

Modern Ethernet

• Based on switches, not multiple access, but still called Ethernet

• We’ll get to it in a later segment

Switch

Twisted pair

Switch ports

• How do wireless nodes share a single link? (Yes, this is WiFi!)

• Build on our simple, wired model

Send? Send?

Wireless Complications

• Wireless is more complicated than the wired case (Surprise!)

1. Media is infinite – can’t Carrier Sense2. Nodes can’t hear while sending – can’t Collision Detect

≠ CSMA/CD

No CS: Different Coverage Areas

• Wireless signal is broadcast and received nearby, where there is sufficient SNR

No CS: Hidden Terminals

• Nodes A and C are hidden terminals when sending to B

• Can’t hear each other (to coordinate) yet collide at B• We want to avoid the inefficiency of collisions

No CS: Exposed Terminals

• B and C are exposed terminals when sending to A and D

• Can hear each other yet don’t collide at receivers A and D• We want to send concurrently to increase performance

Nodes Can’t Hear While Sending

• With wires, detecting collisions (and aborting) lowers their cost

• More wasted time with wireless

Time XXXXXXXXX

XXXXXXXXX

WirelessCollision

ResendX

WiredCollision

Resend

Wireless Problems:

• Ideas?

MACA (Multiple Access with Collision Avoidance) • MACA uses a short handshake instead of CSMA (Karn, 1990)

• 802.11 uses a refinement of MACA (later)

• Protocol rules:1. A sender node transmits a RTS (Request-To-Send, with frame length)

2. The receiver replies with a CTS (Clear-To-Send, with frame length)

3. Sender transmits the frame while nodes hearing the CTS stay silent

• Collisions on the RTS/CTS are still possible, but less likely

MACA – Hidden Terminals

• A B with hidden terminal C1. A sends RTS, to B

DCBARTS

MACA – Hidden Terminals (2)

• A B with hidden terminal C2. B sends CTS, to A, and C too

DCBARTS

CTSCTS

Alert!

MACA – Hidden Terminals (3)

• A B with hidden terminal C3. A sends frame while C defers

Quiet...

MACA – Exposed Terminals

• B A, C D as exposed terminals• B and C send RTS to A and D

DCBARTSRTS

MACA – Exposed Terminals (2)

• B A, C D as exposed terminals• A and D send CTS to B and C

DCBARTSRTS

CTSCTS

All OKAll OK

MACA – Exposed Terminals (3)

• B A, C D as exposed terminals• A and D send CTS to B and C

DCBAFrameFrame

• Assumptions? Where does this break?

802.11, or WiFi

• Very popular wireless LAN started in the 1990s

• Clients get connectivity from a (wired) AP (Access Point)

• It’s a multi-access problem

• Various flavors have been developed over time

• Faster, more features

AccessPoint

Client

To Network

802.11 Physical Layer

• Uses 20/40 MHz channels on ISM (unlicensed) bands• 802.11b/g/n on 2.4 GHz

• 802.11 a/n on 5 GHz

• OFDM modulation (except legacy 802.11b)• Different amplitudes/phases for varying SNRs

• Rates from 6 to 54 Mbps plus error correction

• 802.11n uses multiple antennas• Lots of fun tricks here

802.11 Link Layer

• Multiple access uses CSMA/CA (next); RTS/CTS optional • Frames are ACKed and retransmitted with ARQ (why?)• Funky addressing (three addresses!) due to AP• Errors are detected with a 32-bit CRC• Many, many features (e.g., encryption, power save)

Packet from Network layer (IP)

802.11 CSMA/CA for Multiple Access

• Still using BEB!

Centralized MAC: Cellular

• Spectrum suddenly very very scarce• We can’t waste all of it sending JAMs

• We have QoS requirements• Can’t be as loose with expectations

• Can’t have traffic fail

• We also have client/server• Centralized control

• Not peer-to-peer/decentralized

GSM MAC

• FDMA/TDMA

• Use one channel for coordination – Random access w/BEB (no CSMA, can’t detect)

• Use other channels for traffic• Dedicated channel for QoS

Link Layer: Switching

• How do we connect nodes with a switch instead of multiple access

• Uses multiple links/wires • Basis of modern (switched) Ethernet

Switch

Switched Ethernet

• Hosts are wired to Ethernet switches with twisted pair

• Switch serves to connect the hosts• Wires usually run to a closet

Switch

Twisted pair

Switch ports

What’s in the box?• Remember from protocol layers:

Network

Link Link

Physical PhysicalHub, orrepeater

Switch

Router

All look like this:

Inside a Hub

• All ports are wired together; more convenient and reliable than a single shared wire

Inside a Repeater

• All inputs are connected; then amplified before going out

Inside a Switch

• Uses frame addresses (MAC addresses in Ethernet) to connect input port to the right output port; multiple frames may be switched in parallel

Fabric

Inside a Switch (2)

• Port may be used for both input and output (full-duplex)

• Just send, no multiple access protocol

1 → 4and

2 → 3

Inside a Switch (3)

• Need buffers for multiple inputs to send to one output

. . . . . .

Input Buffer Output BufferFabric

Input Output

Inside a Switch (4)

• Sustained overload will fill buffer and lead to frame loss

. . . . . .

Input Buffer Output BufferFabric

Input Output

Advantages of Switches

• Switches and hubs (mostly switches) have replaced the shared cable of classic Ethernet

• Convenient to run wires to one location• More reliable; wire cut is not a single point of failure that

is hard to find

• Switches offer scalable performance• E.g., 100 Mbps per port instead of 100 Mbps for all nodes

of shared cable / hub

Switch Forwarding

• Switch needs to find the right output port for the destination address in the Ethernet frame. How?

• Link-level, don’t look at IP

. . . . . .

Source

Destination

Ethernet Frame

Switch Forwarding

• Ideas?

. . . . . .

Source

Destination

Ethernet Frame

Backward Learning

• Switch forwards frames with a port/address table as follows:1. To fill the table, it looks at the source address of input frames

2. To forward, it sends to the port, or else broadcasts to all ports

Backward Learning (2)

• 1: A sends to D

Switch

Address Port

• 2: D sends to A

Switch

Address Port

• 3: A sends to D

Address Port

Switch

Learning with Multiple Switches

• Just works with multiple switches and a mix of hubs, e.g., A -> D then D -> A

Switch

Learning with Multiple Switches

• Just works with multiple switches and a mix of hubs, e.g., A -> D then D -> A

Switch

Problems?

Problem – Forwarding Loops

• May have a loop in the topology• Redundancy in case of failures• Or a simple mistake

• Want LAN switches to “just work”• Plug-and-play, no changes to hosts• But loops cause a problem …

Redundant Links

Forwarding Loops (2) • Suppose the network is started and A

sends to F. What happens?

Left / Right

Forwarding Loops (3) • Suppose the network is started and A sends to F.

What happens?• A → C → B, D-left, D-right

• D-left → C-right, E, F

• D-right → C-left, E, F

• C-right → D-left, A, B

• C-left → D-right, A, B

• D-left → …

• D-right → …

Left / Right

Spanning Tree Solution

• Switches collectively find a spanning tree for the topology

• A subset of links that is a tree (no loops) and reaches all switches

• They switches forward as normal on the spanning tree• Broadcasts will go up to the root of the tree and down all

the branches

Spanning Tree (2)

Topology One ST Another ST

Spanning Tree (3)

Topology One ST Another ST

Spanning Tree Algorithm

• Rules of the distributed game:• All switches run the same algorithm

• They start with no information

• Operate in parallel and send messages

• Always search for the best solution

• Ensures a highly robust solution• Any topology, with no configuration

• Adapts to link/switch failures, …

Radia Perlman (1952–)

• Key early work on routing protocols• Routing in the ARPANET

• Spanning Tree for switches (next)

• Link-state routing (later)

• Worked at Digital Equipment Corp (DEC)

• Now focused on network security

Spanning Tree Algorithm (2)

• Outline:1. Elect a root node of the tree (switch with the lowest address)

2. Grow tree as shortest distances from the root (using lowest address to break distance ties)

3. Turn off ports for forwarding if they aren’t on the spanning tree

Spanning Tree Algorithm (3)

• Details:• Each switch initially believes it is the root of the tree• Each switch sends periodic updates to neighbors with:

• Its address, address of the root, and distance (in hops) to root• Short-circuit when topology changes

• Switches favors ports with shorter distances to lowest root• Uses lowest address as a tie for distances

Hi, I’m C, the root is A, it’s 2 hops away or (C, A, 2)

Spanning Tree Example• 1st round, sending:

• A sends (A, A, 0) to say it is root

• B, C, D, E, and F do likewise

• 1st round, receiving:• A still thinks is it (A, A, 0)

• B still thinks (B, B, 0)

• C updates to (C, A, 1)

• D updates to (D, C, 1)

• E updates to (E, A, 1)

• F updates to (F, B, 1)

A,A,0 B,B,0

E,E,0 F,F,0

Spanning Tree Example (2)• 2nd round, sending

• Nodes send their updated state

• 2nd round receiving:• A remains (A, A, 0)

• B updates to (B, A, 2) via C

• C remains (C, A, 1)

• D updates to (D, A, 2) via C

• E remains (E, A, 1)

• F remains (F, B, 1)

A,A,0 B,B,0

E,A,1 F,B,1

Spanning Tree Example (3)• 3rd round, sending

• Nodes send their updated state

• 3rd round receiving:• A remains (A, A, 0)

• B remains (B, A, 2) via C

• C remains (C, A, 1)

• D remains (D, A, 2) via C-left

• E remains (E, A, 1)

• F updates to (F, A, 3) via B

A,A,0 B,A,2

E,A,1 F,B,1

Spanning Tree Example (4)• 4th round

• Steady-state has been reached• Nodes turn off forwarding that is

not on the spanning tree

• Algorithm continues to run• Adapts by timing out information• E.g., if A fails, other nodes forget it,

and B will become the new root

A,A,0 B,A,2

E,A,1 F,A,3

Spanning Tree Example (5)• Forwarding proceeds as usual on the ST

• Initially D sends to F:

• And F sends back to D:

A,A,0 B,A,2

E,A,1 F,A,3

• Initially D sends to F:• D → C-left

• C → A, B

• A → E

• B → F

• And F sends back to D:• F → B

• B → C

• C → D

A,A,0 B,A,2

E,A,1 F,A,3

• Initially D sends to F:• D → C-left

• C → A, B

• A → E

• B → F

• And F sends back to D:• F → B

• B → C

• C → D

A,A,0 B,A,2

E,A,1 F,A,3

Problems?

Link Layer: Software Defined Networking

• How do we scale these networks up?• Answer 1: Network of networks, a.k.a. The Internet• Answer 2: Ah, just kinda hope spanning tree works?

SwitchSwitch Switch

Rise of the Datacenter

Datacenter Networking

Scaling the Link Layer

• Fundamentally, it’s hard to scale distributed algorithms

⚫ Exacerbated when failures become common

⚫ Nodes go down, gotta run spanning tree again…

⚫ If nodes go down faster than spanning tree resolves, we get race conditions

⚫ If they don’t, we may still be losing paths and wasting resources

• Ideas?

Software Defined Networking (SDN)

• Core idea: stop being a distributed system⚫ Centralize the operation of the network

⚫ Create a “controller” that manages the network

⚫ Push new code, state, and configuration from “controller” to switches

⚫ Run link state with a global view of the network rather than in a distributed fashion.

⚫ Allows for “global” policies to be enforced.

⚫ Can resolve failures in more robust, faster manners

⚫ Problems?

SDN – Problem 1

• Problem: How do we talk to the switches if there’s no network?

⚫ Seems a little chicken-and-egg

⚫ Nodes go down, gotta run spanning tree again…

⚫ If nodes go down faster than spanning tree resolves, we get race conditions

⚫ If they don’t, we may still be losing paths and wasting resources

• Ideas?

SDN – Control and Data Planes

SDN – Problem 2

• Problem: How do we efficiently run algorithms on switches?

⚫ These are extremely time-sensitive boxes

⚫ Gotta move the packets!

⚫ Need to be able to support

⚫ Fast packet handling

⚫ Quick route changes

⚫ Long-term policy updates

• Ideas?

SDN – OpenFlow

Control Program A Control Program B

Controller

Packet

Forwarding

Packet

Forwarding

Packet

Forwarding

Table(s)

“If header = p, send to port 4”

“If header = ?, send to me”

“If header = q, overwrite header with r,

add header s, and send to ports 5,6”

SDN – OpenFlow

• Two different classes of programmability

• At Controller

⚫ Can be heavy processing algorithms

⚫ Results in messages that update switch flow table

• At switch

⚫ Local flow table

⚫ Built from basic set of networking primitives

⚫ Allows for fast operation

SDN – Timescales

Data Control Management

Time-scale Packet (nsec) Event (10 msec to sec)

Human (min to hours)

Location Linecard hardware

Router software Humans or scripts

SDN – OpenFlow

Control Program A Control Program B

Controller

Packet

Forwarding

Packet

Forwarding

Packet

Forwarding

Table(s)

“If header = p, send to port 4”

“If header = ?, send to me”

“If header = q, overwrite header with r,

add header s, and send to ports 5,6”

SDN – Key outputs

• Simplify network design and implementation?

⚫ Sorta. Kinda pushed the complexity around if anything

• However...

⚫ Does enable code reuse and libraries

⚫ Does standardize and simplify deployment of rules to switches

⚫ Allows for fast operation

Link Layer - courses.cs.washington.edu

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